The present invention generally relates to a seal ring, a method of manufacturing the same, and a forming die of the same and more particularly relates to a seal ring used for sealing a portion where hydraulic control of automatic transmissions of AT motorcars and the like is required, a method of manufacturing the same, and a forming die of the same.
FIG. 1 shows a conventional seal ring having a cut portion 2 at one place on its circumference.
A conventional injection molding die for forming such a seal ring is shown in FIG. 2. A molten molding material is pressed under high pressure into a cavity 4a through a gate 3 of a forming die 4A and then it is hardened by cooling or heating.
In this case, if the flow length of the molten molding material in the cavity 4a within the forming die is too long, it sometimes occurs that the molding material is not completely filled into the cavity 4a of the forming die. Also, it sometimes occurs that the molding material begins to harden before it fully flows into the cavity 4a of the forming die. In such cases, the molding material is not fully failed in the die and, hence, a work of exactly the same form as the die cannot be produced. Such phenomena are called "short in a moulding" and "sink mark".
Therefore, to make the flow length of the molding material within the cavity 4a of the forming die as small as possible, it was practiced to provide the gate 3 at the portion opposite to the cut portion 12. More particularly, the cut portion 12 and the gate 3 were located along the direction of the diameter 4b of the circular cavity 4a.
When the seal ring 1 is loaded in place, the cut portion 2 is widened to left and right. At that time, a maximum strain is generated at the opposite portion 5 to the cut portion 2.
When the gate 3 is provided at the position opposite to (the other side across the diameter of) the cut portion 12 of the forming die 4A, the seal ring 1 produced by such an arrangement has a lower strength. The reason is that the portion corresponding to the gate 3, i.e., the portion-corresponding-to-gate 13, has somewhat lowered strength compared with other portions of the seal ring 1. As a result, in the case where a material exhibiting smaller elongation is used as the material for the seal ring 1, it tends to break when it is loaded in place because the opposite portion 5 to the cut portion 2 of the seal ring 1 coincides with the portion-corresponding-to-gate 13.
Further, a seal ring 1 can be molded by the use of a forming die 4B having no cut portion as shown in FIG. 3. In this case, the seal ring 1 is cut at the position corresponding to the gate 3 and thus the cut portion 2 is provided. Namely, the portion-corresponding-to-gate becomes the cut portion. In the case of FIG. 3, the flows of the molding material divided into two directions through the cavity 4b at the gate 3 form the so-called weld portion 14 at the position where they meet each other. Since the weld portion 14 does not provide a perfect bond, it has somewhat lowered strength as compared with other portions. Therefore, if a weld portion 14 is formed at the opposite portion 15 to the cut portion 2, the cut portion 2 is widened when the seal ring is loaded in place, and the opposite portion 15 to the cut portion 2 comes to be damaged, especially for a material exhibiting small elongation.
On the other hand, generally, seal rings are molded out of molding materials with PEEK used as the base. PEEK is a resin being hard and having a high melting point Therefore, moldings formed of such a PEEK-based forming material are hard and brittle. It is difficult to make a cut with a knife in a PEEK seal ring after it has been molded. If it is forcibly cut, there arises a possibility of breaking it
Therefore, PEEK seal rings are molded by injecting the molding material into the cavity 4b of a forming die 4A having a cut portion 12 previously provided as shown in FIG. 2. Otherwise, PEEK seal rings are molded by the use of a forming die 4c partly joined at the cut portion 2 as shown in FIG. 4A and FIG. 4B, and the portion 2a enclosed with broken lines is cut off by using a milling machine in a subsequent working process.
Further, the PEEK-based forming material is less fluid at the time of injection. Therefore, troubles of short shot are liable to occur when the flow length is too long or the flow passage area is too small.